EP-A-1456048 describes a transmission system with a continuously variable transmission ratio comprising an electric machine with a stator and two coaxial rotors. The stator is external and the two rotors are arranged concentrically inside the stator. The first rotor is provided with a polyphase winding and the second rotor carries a plurality of magnets that cooperate with the first rotor winding and the stator winding. A first and a second inverter are connected to the first rotor winding and to the stator winding, respectively.
This solution has several drawbacks, including the need for power supply brushes for the first rotor winding and the need to provide a coolant system of the first rotor, which is integrated into the shaft of the rotating rotor.
EP-A-0945963 describes an electric machine comprising a first rotor having a plurality of magnetic poles, a second rotor having a plurality of magnetic poles and supported in a freely rotatable manner, coaxially to the first rotor, and a stator comprising a winding formed by a plurality of coils arranged at equal regular intervals. The stator winding is supplied with a first alternating current, which forms a number of rotating magnetic fields equal to the number of magnetic poles of the first rotor, and with a second alternating current, which forms rotating magnetic fields of a number equal to the number of magnetic poles of the second rotor. A controller supplies the stator winding with a composite electric current comprising the first and the second alternating currents.
The drawback of this solution is that the two rotors are not electromagnetically independent of each other, and it is therefore very difficult to avoid a torque transmission to the second rotor when the stator winding is supplied with the driving current of the first rotor, and vice versa.
EP-A-1673848 describes a double rotor electric machine with axial flux, comprising two disc-shaped rotors arranged on opposite sides of an intermediate stator equipped with a stator winding. Each of the rotors carries four permanent magnets arranged at 90°, with the same polarity on each rotor. In this case as well, the two rotors are not electromagnetically independent of each other regarding the magnetic flux generated by the stator winding.